1. Field of the Invention
The present invention relates to a liquid jet recording head that carries out a recording operation by acting thermal energy on recording liquid and discharges the recording liquid from discharge ports and an apparatus having a recording mechanism using said recording head.
2. Related Background Art
A liquid jet recording head applied to, for example, a liquid jet recording apparatus is generally provided with liquid discharge ports which serve for discharging and ejecting recording liquid to produce flying droplets, liquid passages each communicating with each discharge port and an energy generating means which is provided in a part of the liquid passage and generates energy utilized for obtaining flying liquid droplets from the recording liquid stored in the liquid passage.
Exemplary energy generating means of the above mentioned energy generating means are a pressure energy means represented by an electromechanical converter such as a piezoelectric element, etc., an electromagnetic wave energy generating means for applying electromagnetic wave such as laser, etc., to recording liquid to form flying liquid droplets, or an electrothermal converter, which have been all well-known.
The liquid jet recording head employing a thermal energy generating means such as the above-mentioned electrothermal converter can conduct a recording with high resolution, since the liquid discharge ports used for forming the flying liquid droplets required for recording can be arranged with high density. The miniaturization of the recording head can be easily made. Further, in the process of manufacturing the recording head, the advantages of IC technology or micronization technology which have been recently significantly improved from the viewpoint of reliability and progress in the field of semiconductor can be adopted as much as possible. It is also possible to readily lengthen the size of the recording head or to achieve the planer use thereof (in two-dimensional way). It is noted that, with the above described points considered, that the multi-nozzle formation and high density of the recording head can be attained without difficulty, in addition to that, a large quantity of recording heads can be manufactured with high productivity and at low production cost. The recording head using the above thermal energy generating means is, therefore, worthy of note.
FIG. 1 shows an example of a conventional liquid jet recording head provided with such a thermal energy generating means. The recording head 101 has a structure that electrothermal converters 103 as thermal energy generating means, electrodes 104, liquid passage walls 105 and a top plate 106 are provided on a substrate 102 through processes for manufacturing semiconductors which make use of various processes including etching, vapor deposition, sputtering or the like. Recording liquid 112 is supplied from a recording liquid tank not shown to the common liquid chamber 108 of the recording head 101 through a liquid supply pipe 107. The recording liquid 112 supplied to the common liquid chamber 108 is supplied to liquid passages 110 in accordance with for example, capillary phenomenon and forms a meniscus in each liquid discharge port 111 placed at the end of each liquid passage 110, so that it is stably retained.
To discharge the recording liquid by utilizing the recording head 101 constructed as above, for instance, the electrothermal converters 103 are energized in the form of pulsation. As a result of the energization of the electrothermal converters 103, the recording liquid 112 located in the vicinity of the electrothermal converters 103 is rapidly heated. The rapid heating thereof produces foaming phenomenon in the recording liquid 112. The foaming energy generated from the foaming phenomenon enables liquid droplets to be discharged from the liquid discharge ports 111. With reference to the liquid jet recording head 101 using such thermal energy generating means, a liquid jet recording head can be obtained with ease and high productivity that has, for example, a construction mentioned above and the arrangement of discharge ports provided with such a high density as 16/mm for example, and that is a multi-nozzle type having 128 or 256 nozzles.
The liquid jet recording head of multi-nozzle type in which the thermal energy generating means such as the aforementioned electrothermal converters are disposed with high density is generally designed so that the length of each liquid passage on the substrate 102, the cross-sectional area of each liquid passage, each liquid discharge port area, the distance between each liquid discharge port and each electrothermal converter, the heating area of each electrothermal converter or the like are respectively equal in every nozzle of the multiple nozzles in order to maintain the uniformity of liquid droplets emitted from the respective liquid discharge ports.
However, the liquid jet recording head provided with the above construction, is, during recording operation by the head, liable to diffuse more heat at both ends than at a central portion with respect to a direction where the liquid passages are arranged, which leads to the generation of a temperature gradient in the substrate 102. Accordingly, as shown in FIG. 2, it is apparent that the temperature at the end regions of the multi-nozzle type substrate is inclined to become lower than the temperature at the central region thereof.
As an exemplary recording liquid for use with the liquid jet recording head, a product obtained by, for example, dissolving or dispersing coloring agent such as dyestuff, pigment, etc., into aqueous or oily liquid is typically used. It is well-known that the viscosity of the recording liquid is greatly dependent on the temperature of the recording liquid, regardless of the recording liquid used. FIG. 3 illustrates an example of the dependency of viscosity of the recording liquid on the temperature thereof used in a liquid jet recording head. As apparent from FIG. 3 the viscosity of the recording liquid declines as the temperature thereof rises. In contradistinction thereto, as the temperature of the recording liquid declines, the viscosity thereof rises. As described above, the generation of temperature gradient in the substrate thus causes the viscosity of ink to be decreased at the central portion of the recording head where temperature is high. On the contrary, the viscosity of ink is higher near the two ends where temperature is low.
The phenomenon that the higher the temperature of the recording liquid rises, the lower the viscosity thereof becomes does not merely indicate that the viscosity of the recording liquid changes. For example, even though thermal energy applied to recording liquid from each thermal energy generating means is the same, the volume of liquid droplet emitted from each liquid discharge port 111 is increased or decreased or recording liquid low in viscosity inevitably produces a broader expansion of recording dot, that is, larger dot area on a material to be recorded than that produced using a recording liquid high in viscosity as shown in FIG. 4, when the droplets stick to and are deposited on the material to be recorded such as paper to form the recording dot. According to the above phenomenon, recording cannot be done with desired and stable density.
Namely, when the aforementioned temperature gradient is produced in the substrate 102 in the multi-nozzle type liquid jet recording head and the temperature at both end regions is lower compared with that at a central region, an inconvenience arises in that the viscosity of recording liquid in the central region of the liquid passage 110 is decreased in comparison with the liquid in the end regions and the volume of liquid droplets discharged from the liquid discharge ports 111 at the central region and the dot area of the recording liquid where the discharged droplets come into contact with a material to be recorded and are recorded become larger than those at the end regions.
FIG. 5 shows a graph designating the relation between the recording density of recording liquid emitted from each discharge port 111 and a substrate position when the temperature gradient appears in the substrate 102 of the conventional liquid jet recording head.
Such unevenness in recording density especially appears where all the thermal energy generating means of the recording head are heated and the head repeatedly scans and reciprocates on a material to be recorded such as a recording sheet perpendicularly to the feeding direction of the recording sheet as if the entire surface of the recording sheet were completely painted. In this case, the unevenness in recording density is, as illustrated in FIG. 6, repeatedly produced for every line, and accordingly, variable densities are undesirably repeatedly distinguished.